Separator membranes are important components of batteries. These membranes serve to prevent contact of the anode and cathode of the battery while permitting electrolyte to pass there through. Additionally, battery performance attributes such as cycle life and power can be significantly affected by the choice of separator. Safety can also be related to separator attributes, and certain separators are known to reduce occurrence of Li metal plating at the anode and even dendrite formation.
Separator membranes of battery cells are, in some instances, formed from bodies of porous polymer materials. In other instances, separator membranes are formed from bodies of fibrous or particulate material, and such materials can include glass fibers, mineral fibers such as asbestos, ceramics, synthetic polymeric fibers as well as natural polymeric fibers such as cellulose.
There are a number of problems with the presently utilized separator membranes. Such membranes materials are often expensive, and given the fact that a typical battery system will include relatively large volumes of membranes, the cost of the membranes can be a significant component of overall battery costs. Typical separator membranes used in prior art lithium ion cells are made from polymers such as polyethylene or polypropylene, and they may be fabricated in either a wet or a dry process. In a wet process, polymers of differing molecular weights and an oil are first blended, and then melt extruded to yield a film. The film is subsequently subjected to an extraction (“wet”) step, in which the oil/low molecular weight polyolefins are extracted from the higher molecular weight solid film to leave a porous film. In the dry process for a three layer film, separate layers of polymer film are laminated, drawn down, and annealed so as to provide a polymer structure which has oriented crystallites. The sheet is then rapidly uniaxially stretched to obtain porosity. A similar process is used for dry processing of single layer films. These processes are relatively expensive, and membranes produced thereby have costs in the range of several dollars per square meter. The high cost of the separators translates to high cost for finished cells. Any reduction in the cost of the membrane will translate to significant savings in the overall cost of batteries. In addition, polymer separators must maintain their size and shape as temperatures are increased beyond the usual operating temperatures, to assure continued physical separation between anode and cathode. Many separators shrink unacceptably at increased temperatures and unacceptably allow the two electrodes to contact each other and thereby causing the cell to rapidly discharge, further contributing to unsafe increases in cell temperature. It is an important safety feature for the separators to maintain shape and original size and to avoid electrode contact at high temperatures.
Inorganic composite materials have also been used as separators. Such composite separators include a silica (or other ceramic) filler material and a polymer binder. The filler and binder are blended and extruded to form a composite sheet and any volatile components are removed by extraction or evaporation to form a porous body. Other examples blend the filler and binder to form a mixture that is applied to a substrate by various coating means, such as doctor blading, roll coating or screen, stencil printing or gravure. In many cases, the composite separator materials contain a very high content of inorganic filler. In some instances, the separators exhibit poor properties, such as mechanical properties.
Low cost battery separator membrane materials can be inefficient in preventing dendrite bridging, and hence must be made relatively thick. However, this thickness increases the internal resistance of the battery thereby decreasing its efficiency, and also increases battery size. In addition, various separator materials are fragile, and this fragility can complicate the manufacture of battery systems and both increase cost to manufacture a cell and potentially compromise safety.
Thus, there is a need for separator membranes which are efficient, low in cost, safe and easy to utilize.